Use direct backend URLs for inventory API and guard JSON parsing

frontend/src/lib/inventoryApi.ts

@@ -4,17 +4,22 @@ async function handleResponse<T>(response: Response): Promise<T> {
   if (!response.ok) {
     throw new Error(`API error: ${response.status}`);
   }
+  const contentType = response.headers.get('content-type') || '';
+  if (!contentType.includes('application/json')) {
+    const text = await response.text();
+    throw new Error(`API returned non-JSON response: ${text.slice(0, 80)}`);
+  }
   return response.json() as Promise<T>;
 }
 
 export async function fetchInventoryTree(): Promise<InventoryTreeNode[]> {
-  const response = await fetch('/inventory/tree');
+  const response = await fetch('http://127.0.0.1:8000/inventory/tree');
   const data = await handleResponse<ListResponse<InventoryTreeNode>>(response);
   return data.items;
 }
 
 export async function fetchNodeContext(id: string): Promise<NodeContext> {
-  const response = await fetch(`/inventory/nodes/${id}/context`);
+  const response = await fetch(`http://127.0.0.1:8000/inventory/nodes/${id}/context`);
   const data = await handleResponse<EntityResponse<NodeContext>>(response);
   return data.item;
 }